1. Field of the Invention
This invention relates to a device and method for removing toner dispersant from an image formed from a liquid developer.
2. Description of the Prior Art
A typical electrostatographic printing machine (such as a photocopier, laser printer, facsimile machine or the like) employs an imaging member that is exposed to an image to be printed. Exposure of the imaging member records an electrostatic latent image on it corresponding to the informational areas contained within the image to be printed. The latent image is developed by bringing a developer material into contact therewith. The developed image recorded on the photoconductive member is transferred to a support material such as paper, either directly or via an intermediate transport member. The developed image on the support material is generally subjected to heat and/or pressure to permanently fuse it thereto.
Two types of developer materials are typically employed in electrostatographic printing machines. One type of developer material is known as dry developer material and comprises toner particles or carrier granules having toner particles adhering triboelectrically thereto. Another type of developer material is a liquid material comprising a liquid carrier or dispersant having toner particles dispersed therein.
When using a liquid developer material, excess liquid carrier such as Isopar (a non-polar decane solvent) frequently adheres to the photoconductive member and is transferred to the transport member (if any) and support material. This liquid carrier later evaporates into the air. Usually about 0.5 grams of liquid carrier is absorbed by the copy paper and carried out in each copy.
As the image is fused to the paper by heating the toner above its melting point and allowing it to flow into the pores of the paper, the heat required to fuse the image vaporizes a large percentage of the liquid carrier. It is desirable to remove the liquid carrier from the support material to minimize image show through and to prevent problems associated with it emerging from support material later. Additionally, increasing the solids content of an image before transferring greatly improves the ability of the toner particles to form a high resolution image on the transport member and thus on the support material.
The amount of dispersant in the toned image can be limited by a metering system. The most common metering technique is the reverse roll.
Reverse roll doctoring and corona doctoring reduce the amount of liquid carried out by the copy sheet from about 0.5 grams to about 0.12 grams per copy. Reverse roll doctoring provides superior background clean-up by having sufficient shear force to remove all the liquid carrier except the liquid carrier electrostatically bonded to the toner particles. However, very close spacing is required to do an effective job. It is particularly difficult to maintain this close spacing over large dimensions in applications such as color proofing masters and other graphic arts.
An air knife can also remove excess liquid carrier. However, the toner particles adhering to the latent image may also be removed, thereby disturbing the image.
Various other techniques have been devised for removing excess liquid carrier from an imaging member.
U.S. Pat. No. 3,955,533 (Smith et al.) discloses a squeegee roller system for removing excess developer liquid from the developer-image-bearing photoconductive surface of a drum or the like. The squeegee roller has a covering of a predetermined hardness which is biased with a predetermined force against the moving surface carrying a developed image to cause excess developer to flow into a receptacle.
U.S. Pat. No. 4,089,683 (Knieser) discloses a liquid developer cleaning means which includes a foam-backed belt having a working surface pattern of ridges and grooves. The foam-backed belt is mounted on two guide rolls rotatable about a shaft and positioned such that the movable belt advances during operation in a direction of an axis upon which a drum having an image surface thereon rotates. The purpose of the belt is to clean any residual developer after the toner laden image has been transferred from the drum.
U.S. Pat. No. 4,258,115 (Magome et al.) discloses a wet developing method which includes the use of an elastic roller for collecting excess developing liquid and thereby effecting further development of an electrostatic image at the development station. The elastic roller is comprised of a shaft formed of a rigid material, an elastic foam member surrounding the shaft, and a netting which is supported around the foam member. The elastic roller may be in the shape of a belt.
U.S. Pat. No. 4,263,391 (Saito et al.) discloses a liquid development process which includes a transfer step, and a cleaning step for cleaning a surface of a latent image carrying member. An elastic rotary member may be used to clean the latent image carrying member. The elastic rotary member is made of an electroconductive rigid core member, an electroconductive porous elastic member capable of retaining a liquid therein and provided around the core member, and a liquid-permeable insulating flexible member surrounding an outer periphery of the elastic member. The elastic rotary member is maintained in pressure contact with the latent image carrying member. The liquid absorbed may be squeezed out through the meshes of the elastic member when the elastic member is compressed.
U.S. Pat. No. 4,299,902 (Soma et al.) discloses an image forming process which utilizes an elastic roller or belt which squeezes out liquid developer and then absorbs excess liquid developer. The elastic roller or belt is used to apply developer to a latent image and absorb excessive liquid developer. Part of the roller or belt is kept in a liquid developer reservoir.
In U.S. Pat. No. 4,985,733 (Kurotori et al.) an image fixing unit for a wet-type electrophotographic copying machine comprises a blotter roller which absorbs a carrier liquid component, such as Isopar, contained in a developer deposited imagewise by an image-transfer charger on a transfer sheet. The blotter roller is comprised of a porous material such as a non-woven fabric or cotton and an elastic material such as silicone rubber. The transfer sheet is transported into an image fixing unit wherein a pressure-application roller is in contact with a heat-application roller, such that a toner image formed on the transfer sheet is thermally fixed thereto, while the transfer sheet passes through a nip between the heat-application roller and the pressure-application roller. The blotter roller is in contact with a back-up roller with a predetermined pressure. A cleaning brush roller may be used to clean the blotter roller.
U.S. Pat. No. 5,028,964 (Landa et al.) discloses an apparatus for image transfer which comprises an intermediate transfer member and a squeegee for removing excess liquid from the toner image prior to transferring an image. The intermediate transfer member is operative for receiving the toner image therefrom and for transferring the toner image to a receiving substrate. Transfer of the image to the intermediate transfer member is aided by providing electrification of the intermediate transfer member to a voltage having the same bias as that of the charge particles.
U.S. Pat. No. 4,286,039 (Landa et al) discloses an image forming apparatus comprising a deformable polyurethane roller, which may be a squeegee roller or blotting roller which is biased by a potential having a sign the same as the sign of the charged toner particles in a liquid developer. The bias on the polyurethane roller is such that it prevents streaking, smearing, tailing or distortion of the developed electrostatic image and removes much of the liquid carrier of the liquid developer from the surface of the photoconductor.
U.S. Pat. No. 4,392,742 (Landa) discloses a cleaning system for a liquid developer electrostatographic copier comprising a roller formed with a resilient material, such as a closed-cell elastomer, having externally exposed, internally isolated surface cells. During an operation, the excess liquid on an imaging surface is absorbed by the cleaning roller. The cleaning roller is then compressed to squeeze out liquid from the roller, leaving the roller dry.
Each of these methods and devices have certain limitations. Some of the devices such as that of U.S. Pat. No. 4,089,683 clean up the residual developer only after the toner laden image has been transferred from the drum. Some of the other methods and devices discussed above serve to collect and distribute excess developing fluid over the electrostatic image, and do not remove excess and unwanted developing liquid. Other systems use rollers to absorb excess developer liquid from the developer image bearing photoconductive surface of the drum.